This invention relates generally to synthetic materials. More specifically, the invention relates to a multi-phase material comprised of a matrix of transition metal-based ceramic having one or more specific metals or semiconductors present as a nanodispersed phase therein. The invention also relates to electrodes incorporating these materials, and in particular to anodes for rechargeable lithium batteries.
Rechargeable batteries are of ever increasing importance as power supplies for a variety of items. One important class of rechargeable batteries comprises rechargeable lithium batteries, and as used herein, the term is understood to include all types of rechargeable lithium and lithium ion batteries.
The anode is an important component of a lithium battery. During charging and discharging of the battery, lithium ions are inserted into, and removed from, the bulk of the anode material respectively. The performance and practicality of a lithium battery will depend, in a significant part, upon the properties of the materials comprising its electrodes. Anode materials for lithium batteries should have high charge capacity, and should provide a good rate of charge and discharge. In addition, the materials should have good stability with regard to both operational cycling and ambient conditions. Also, in order to be practical, the materials should be relatively low in cost.
Presently, anodes for rechargeable lithium batteries are generally graphite-based. Graphite is relatively low in cost; however, many of its performance characteristics are less than ideal. For example, graphite has relatively low volumetric capacity. As is understood in the art, volumetric capacity is a measure of the charge storing ability of a material on the basis of a given unit volume. As will be explained in detail herein below, the materials of the present invention have a volumetric capacity which is at least 3 to 5 fold greater than that of prior art graphite-based materials. It is also significant that the diffusion coefficient of lithium ions is far greater in the materials of the present invention than in graphite. Because of this high diffusion coefficient the discharge rate for batteries made in accord with the present invention is high, as compared to those of the prior art. Discharge rate is particularly important in applications requiring instantaneous high power such as radio and cell phone broadcasting, power tools, and the like.
A significant problem of graphite-based electrodes is that lithium can plate out on to the graphite material under overcharge conditions. This plating out can damage or destroy a battery in which it occurs, and can also present a significant hazard. The materials of the present invention have been found to be very resistant to plate out of lithium, even under relatively high overcharge conditions.
As will be explained in detail herein below, the multi-phase materials of the present invention include a matrix comprised of a transition metal-based ceramic, and the lattice structure of this matrix provides very good mechanical and chemical stability. As a result, the materials of the present invention inhibit migration of metal domains, and cells incorporating these materials manifest very low first cycle losses. As is understood in the art, first cycle loss refers to degradation of cell performance, in terms of capacity and discharge rate, which occurs as a result of the first charge/discharge cycle. This first cycle loss can be relatively large in cells of the prior art; and consequently, a predetermined amount of overcapacity must be built into prior art cells to account for first cycle losses. Since cells incorporating the electrode materials of the present invention have low first cycle losses, they do not require large overcapacities. In addition to operational stability, the materials of the present invention are quite stable to ambient atmospheric conditions; hence, design criteria, and manufacturing processes for cells incorporating the present materials are greatly simplified.
In addition to the foregoing, the materials of the present invention are manufactured from relatively low-cost starting materials. These and other advantages of the present invention will be apparent from the drawings, discussion and description which follow.
There is disclosed herein a multi-phase material comprising a first phase which is a ceramic matrix material comprised of a compound of a group IV-VI transition metal. The compound is selected from the group consisting of nitrides, carbides, oxides, and combinations thereof. The material includes a second phase comprising a material selected from the group consisting of Sn, Sb, Bi, Pb, Ag, In, Si, Ge, and combinations thereof. This second phase is present as a nanodispersion in the matrix. In some embodiments, the second phase preferably includes tin, and may further include one or more alloying elements such as antimony. In some embodiments, the transition metal comprises Ti or V. The nanodispersed phase is typically present as regions having a size in the range 2,000-5,000 angstroms.
In some embodiments, the material is of the general formula: Tx, A1xe2x88x92x, By, Oz, wherein T is one or more group IV-VI transition metals, A is selected from the group consisting of Sn, Sb, Bi, Pb, Ag, In, Si, Ge, and combinations thereof; B is carbon, nitrogen, or combinations thereof, and O is oxygen. The subscripts refer to atomic percentages of the various components, and x is in the range of 0.4-0.6; y is in the range of 0.2-0.6; z is in the range of 0.0-0.3, and the sum of y+z is in the range of 0.8x-1.2x. In one particularly preferred embodiment, A is a mixture of tin and antimony, and the antimony is present in the range of 0.01-0.05 atomic percent of the total material.
The present invention also includes anodes for lithium batteries which incorporate the foregoing materials.